Overvoltage arrestor with a discharge in a low temperature atmosphere



Apnl 10, 1956 P. LEMAlGRE-VOREAUX 2,741,680

OVERVOLTAGEI ARRESTOR WITH A DISCHARGE IN A LOW TEMPERATURE ATMOSPHERE Filed Nov. so, 1954 I NVE NTOR PIERRE LEMAIGREVOREAUX BY M a? United States PatentOo OVERVOLTAGE ARRESTOR WITH A DISCHARGE IN A LOW TEMPERATURE ATMOSPHERE Pierre Lemaigre-Voreaux, Paris, France, assignor to Societe Anonyme Pour les Applications de lElectricite et des Gas Rares-Etablissements Clantieda; & Silva, Paris, France Application November 30, 1954, Serial No. 472,030

Claims priority, application France December 2, 195 3 2 Claims. (Cl. 200-1135) This invention relates to an overvoltage arrestor of the kind comprising a vacuum-tight envelope containing a low-pressure atmosphere, a first and a second electrodes and a first and a second contact parts, said first electrode comprising a bimetallic portion mechanically and electrically connected with said first contact part, said second electrode being electrically connected with said second contact part, said second contact part being located in the path travelled through by said first contact part when said bimetallic portion is heated up.

It is known that the fact of providing a discharge type overvoltage arrestor with a bimetal strip which short circuits its electrodes when it has to pass a sufiiciently heavy overvoltage, will result in a decrease in the amount of heat which is evolved during the passage of such overvoltage.

One object of the invention is to provide an overvoltage arrestor of the kind specified having features which give a more thorough security to the equipment where use is made of this arrestor.

One feature of the invention is to constitute the contact parts with a metal comprising mainly iron, or nickel,

r iron and nickel.

Another feature of the invention is the atmosphere of the arrestor, which comprises one rare gas or a plurality of rare gases, constituting about 99.5% of the atmosphere, the remainder being nitrogen.

An optional but advantageous feature of the invention is to shape and arrange the contact parts so that their contact area, when the bimetallic portion of the first electrode has brought them into contact, is very small.

I have found that with an arrestor constructed in accordance with the invention there is a tendency for contacts to weld together when the overvoltage arrestor opcrates, a tendency which is at first so small as to be negligible but which becomes more marked as the number of V operations of the apparatus increases. The welding of the contacts puts the overvoltage arrestor out of commission by setting up a permanent short circuit between two of its terminals. After the overvoltage arrestor operation which caused the welding, the replacement of a new fuse in the line which supplies the installation to be protected will merely result in the immediate melting of such new fuse so that one cannot help noting that the arrestor is out of commission. During all that time, the installation to be protected remains in safety since the blowing of the fuse insulates it from the line feeding it.

It happens sometimes, in the case of the usual overvoltage arrestors of the type specified, that the arrestor is damaged, either by a variation of its gaseous atmosphere, caused, for instance, by the absorption of part of that atmosphere by the action of discharges, or by a lack of tightness due to a crack in its casing, for example in the vicinity of a current lead-in, which causes it to fill with air. The starting voltage of the arrestor then varies. If the starting voltage drops too much, the arrestor may operate prematurely which would cause unjustified cessations of service. If on the other hand the starting voltage 2,741,680 Patented Apr. 10, 1956 increases much, as a consequence of an entrance of air, the overvoltage arrestor will then only operate at overvoltages which damage the installation to be protected so that the latter is, therefore, no longer protected.

The probability of damage of the overvoltage arrestor increases with the number of operations to which it is subjected. Thus the safety of the protecting device is considerably increased by limiting, by a delayed welding of the electrodes, the number of operations of the overvoltage arrestor.

'The use, in a bimetal strip type overvoltage arrestor, of contact parts made of iron or nickel or of an alloy containing mainly such metals and the employment of an atmosphere of rare gas containing nitrogen in a proportion of the order of 0.5%, makes it possible to obtain the welding of the electrodes after several thousands of operations at the maximum overvoltage for which the arrestor has been designed. This number of operations before welding takes place gives the apparatus, practically quite a sufficient life, shorter, however, than that at which a deterioration could take place.

The use of the additional feature of providing very small contact areas makes it possible, almost certainly, to ensure that welding will take place after a certain number of operations.

In drawings, which illustrate an embodiment of the invention, Figure l is an elevational view, Figure 2 shows a section on the line 11-11 of Figure 1.

Referring to the drawings, it will be seen that the overvoltage arrestor represented comprises an envelope 3 formed by a glass tube which is closed at its upper part by a pointed portion 1, the latter originally forming a part of the usual exhaust tube by means of which a vacuum was obtained in the arrestor and a suitable atmosphere subsequently introduced therein. The envelope 3 is closed at its lower portion by a pressed portion 7 in which are sealed the current lead-in wires 6 and 9 for the two electrodes of the overvoltage arrestor.

The current lead-in wire 6 is welded to the end of a first electrode 2 which consists, for example, of a nickel rod. A glass tube 4, welded at its lower end to the portion 7, surrounds that part of the lead-in wire 6 which is located above said portion 7 as well as the lower portion of the electrode 2.

The tube 4 is heated to softening point to cause it to weld with the wire 6 as the latter, with a view to its sealing in the portion 7, is made out of a metal which welds easily with glass. For example the wire 6 may comprise a composite wire known commercially as Dumet. As a result of this welding, the tube 4 will remain in position even if it should break, for example owing to a shock to the over-voltage arrestor. The tube 4 is generally not welded to the electrode 2, as the metal of the latter is not selected especially with a view to its welding with glass.

The current lead-in wire 9 is butt-welded to a nickel rod 10 which extends substantially parallel to the electrode 2. The place where the wires 9 and 10 are welded to each other is located inside the portion 7. The rod 10, being made of nickel, is not sealed tightly in the portion 7, but the lead-in wire 9 is made of Dumet and hence its sealing in the portion 7 will ensure effective and vacuumtight sealing of the conductors 9, 10.

A bimetallic strip 11 is welded at its lower part to the rod 10. To the upper part of the strip 11 a short piece 12 a few millimeters long of a nickel rod is attached by welding, the axis of said piece being substantially at right angles to that of the electrode 2. The piece 12 forms a contact part. When the bimetallic strip 11, heated by a discharge inside the overvoltage arrestor, is deformed and brings the contact part 12 against the electrode 2, the contact surface between the two latter parts is very small which facilitates their sticking together by welding. The

fact that the contact part 12 and rod 2, which also forms a contact part, are made of nickel, also facilitates the welding of said parts. This has been shown by tests which have also established that similar results may be obtained with contact parts made of metal-s containing mainly iron and nickel. It is to be noted that all metals with an iron or nickel base or an iron and nickel base do not offer this advantage but experience shows easily whether a pair of contact parts are suitable or not.

The second electrode of the overvoltage arrestor is constituted by the wire 10, the bimetallic strip 11 and the part 12.

The atmosphere in the overvoltage arrestor contains 99.4% argon and 0.6% nitrogen under a pressure of 40 mm. of'mercury. When the arrestor is cold the dis tance between the part 12 and electrode 2 is 0.5 mm., the starting voltage of such an arrestor being approximately 225 volts in direct current. Other combinations of interelectrode distance and nature and pressure of the atmosphere may be used for giving either the same starting voltage or different voltages but in all cases, however, the atmosphere should consist of a rare gas and a little nitrogen if the sticking of the electrodes together is desired after a limited number of operations.

Out of 20 overvoltage arrestors of the kind above described, 6 had their electrodes stuck together between 2,500 and 10,000 operations, and all of them between 2,500 and 50,000 operations, caused by the maximum chanically and electrically connected with said first contact part, said second electrode being electrically connected with said second contact part, said second contact part being located in the path travelled through by said first contact part when said bimetallic portion is heated up; the metals of said first and said second contact parts comprising mainly at least one metal selected from the group consisting of iron and nickel, and said atmosphere comprising nitrogen in a proportion of the order of 0.5%, the remainder being comprised of at least one rare gas.

2. An overvoltage arrestor, comprising a vacuum-tight envelope containing a low-pressure atmosphere, a first and a second electrodes and a first and a second contact parts, said first electrode comprising a bimetallic portion mechanically and electrically connected with said first contact part, said second electrode being electrically connected with said second contact part, said second contact part being located in the path travelled through by said first contact part when said bimetallic portion is heated up; the metals of said first and said second contact parts comprising mainly at least one metal selected from the group consisting of iron and nickel, said atmosphere comprising nitrogen in a proportion of the order of 0.5%, the remainder being comprised of at least one rare gas, and the contact area of said contact parts when the bimetallic portion being warmed-up makes the first contact part touch the second contact part, being very small.

References Cited in the file of this patent UNITED STATES PATENTS 2,502,203 Carpenter et a1. Mar. 28, 1950 2,650,278 Foulke Aug. 25, 1953 FOREIGN PATENTS 627,534' Great Britain Aug. 10, 1949 

1. AN OVERVOLTAGE ARRESTOR, COMPRISING A VACUUM-TIGHT ENVELOPE CONTAINING A LOW-PRESSURE ATMOSPHERE, A FIRST AND A SECOND ELECTRODES AND A FIRST AND SECOND CONTACT PARTS, SAID FIRST ELECTRODE COMPRISING A BIMETALLIC PORTION MECHANICALLY AND ELECTRICALLY CONNECTED WITH SAID FIRST CONTACT PART, SAID SECOND ELECTRODE BEING ELECTRICALLY CONNECTED WITH SAID SECOND CONTACT PART, SAID SECOND CONTACT PART BEING LOCATED IN THE PATH TRAVELLED THROUGH BY SAID FIRST CONTACT PART WHEN SAID BIMETALLIC PORTION IS HEATED UP; THE METALS OF SAID FIRST AND SAID SECOND CONTACT PARTS COMPRISING MAINLY AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF IRON AND NICKEL, AND SAID ATMOSPHERE COMPRISING NITROGEN IN A PROPORTION OF THE ORDER OF 0.5%, THE REMAINDER BEING COMPRISED OF AT LEAST ONE RARE GAS. 